Unlike conventional secondary batteries, the redox flow battery is a system in which active material in electrolyte solution is oxidized and reduced and thus charged and discharged, and is an electrochemical storage device that stores the chemical energy of the electrolyte solution directly as electrical energy. Such battery has advantages of being easy to use for large capacity power storage, having high energy density and efficiency, having a long life and being safe. In addition, the battery has attracted attention as a next generation large capacity storage device since the battery has a low maintenance cost because it does not need to be frequently replaced, it operates at room temperature, and it has an advantage that various capacity and output can be designed in various ways.
The basic structure of the redox flow battery includes an electrolyte solution tank in which active materials having different oxidation states to each other are stored, and a pump for circulating it, together with a stack including a structure of bipolar plate/electrode/ion exchange membrane/electrode/bipolar plate.
Actual electrochemical reactions occur in the stack and operate by continuously circulating the electrolyte solution into the stack using a pump. The redox pairs used as the active material in the electrolyte solution include V/V, Zn/Br, Fe/Cr and Zn/air. Among them, V/V and Zn/Br redox pairs are most widely used.
The electrochemical reaction is determined by the interaction between the electrolyte solution flowing along the bipolar plate in the stack and the electrode.
FIG. 1 is a cross-sectional view showing contact between the bipolar plate and the electrode according to the prior art wherein the bipolar plate 11/electrode 12/ion exchange membrane 13/electrode 14/bipolar plate 15 are laminated from the top. This structure is advantageous in that the structure is simple because the electrolyte solution flows directly to the bipolar plates 11 and 15. However, when the charging/discharging is performed under high power conditions or when the size of the battery is increased, there is a problem that as the electrolyte solution is accompanied by a high flow rate, a high differential pressure between the inlet and the outlet of the electrolyte solution is generated, thereby resulting in enormous energy loss.
As an attempt to solve this problem, the structure in which a flow path can be formed inside the bipolar plate and the electrolyte solution flows through the flow path was proposed.
U.S. Patent Application Publication No. 2012-0244395 proposes the structure of the bipolar plates having an interdigitated flow path.
FIG. 2 is a cross-sectional view showing contact between the bipolar plates and an electrode proposed in U.S. Patent Publication No. 2012-0244395, wherein bipolar plate 21/electrode 22/ion exchange membrane 23/electrode 24/bipolar plate 25 are laminated from the top, and flow paths 27 and 29 are formed in the bipolar plates 21 and 25, respectively. When using the bipolar plates of this type, the differential pressure between the inlet and the outlet applied to the battery module was reduced to some extent.
However, referring to the battery of FIG. 2, there is a problem that the contact area between the electrode and the electrolyte solution was reduced due to the configuration of the flow path in contrast to the battery of FIG. 1. As a result, there is a problem that the electrochemical reaction for the generation of electricity is not sufficiently performed, thereby reducing the charging/discharging capacity and the speed of the battery.
In addition, there is a problem that even if it has a flow path structure to reduce internal differential pressure, there is a certain limit in stably controlling the flow rate of the electrolyte solution over all ranges, and the electrolyte solution is retained in the flow path for a short time, thereby failing to ensure sufficient reaction time.
Therefore, there is a growing demand for redox flow batteries that can ensure sufficient reaction time and improve charging/discharging capacity and efficiency, while the energy loss can be minimized regardless of flow rate when the electrolyte solution passes through the flow path.